Power transmissions by chains are widely used in various vehicles, e.g., bicycles and motorcycle. The sprockets in these chain-transmissions transmit the torque from the axels to the chain by the sprocket teeth. For high performance sprockets, two requirements are important. (i) The sprockets must sustain high torque. (ii) The sprockets must be lightweight, for the weight in vehicle would increase the burden on other part of the vehicle, e.g., the engine and the sprocket teeth.
It is not a new idea to use two different materials for the central portion and the peripheral portion of the sprockets, so that lightweight but relatively soft materials can be used at the central portion of the sprockets and harder but heavier materials can be used at the peripheral portion comprising teeth. U.S. Pat. No. 969,237, which was patented in 1910, is one of the examples.
The basic design, which is composed of a central portion and a peripheral portion comprising teeth is widely used. See also U.S. Publication No.: US 2005/0272546.
In certain applications of the two-material design, e.g., U.S. Pat. No. 7,481,729, as well as U.S. Pat. No. 969,237, the peripheral portion's perforated inward extensions, i.e., the toothed portion, that overlap with the central portion with corresponding recesses and holes, act as connection between the central portion and the peripheral portion of the sprocket. That design creates (i) the weak point in the design, i.e., the necks of the extensions, and (ii) the compromise between the strength of the connection and the weight, i.e., the increased number of extensions increases the connective strength but also increases the weight of the sprocket, while the decreased number of extensions increases the force on each extension and thereby increases the metal fatigue at the necks of the extensions.
As the weight of the material increases proportional to the square of the diameter, the thickness of the peripheral portion, i.e., the portion of the diameter that is occupied by the peripheral portion, which is at the outer limit of the sprocket, is critically related to the weight of the overall sprocket. A slight increase in the thickness of the peripheral portion would increase the weight of the entire sprocket significantly.